Subject: How does the 'drinking bird' toy work ?

At home, I have got an astonishing little toy that appearantly 
violates energy conservation. A "bird" consists mainly of a glass tube 
which has a liquid reservoir at its lower end. Its upper end (head)is 
made of a porous material. When the head is weted with some water, the 
liqiud level in the tube starts to raise an evenually the whole bird 
tilts and takes another sip. So, the bird keeps on moving on and 
on. (Sorry, I find it quite difficult to describe the bird but maybe 
it is well know. Otherwise I can send a picture.)
How can the evaporating water produce low pressure and thus make the 
inner liquid raise ?

Re: How does the 'drinking bird' toy work ?